Look in all panes for existing preview to show

Previously only the next pane was checked for an existing
preview which would fail to locate any existing previews
that were moved to different panes or were no longer in the
next pane for the edit session.

src/packages/markdown-preview/lib/markdown-preview.coffee

@@ -15,12 +15,19 @@ module.exports =
       console.warn("Can not render markdown for '#{editSession.getUri() ? 'untitled'}'")
       return
 
-    if nextPane = activePane.getNextPane()
-      if preview = nextPane.itemForUri("markdown-preview:#{editSession.getPath()}")
-        nextPane.showItem(preview)
-        preview.fetchRenderedMarkdown()
-      else
-        nextPane.showItem(new MarkdownPreviewView(editSession.buffer))
+    {previewPane, previewItem} = @getExistingPreview(editSession)
+    if previewItem?
+      previewPane.showItem(previewItem)
+      previewItem.fetchRenderedMarkdown()
+    else if nextPane = activePane.getNextPane()
+      nextPane.showItem(new MarkdownPreviewView(editSession.buffer))
     else
       activePane.splitRight(new MarkdownPreviewView(editSession.buffer))
     activePane.focus()
+
+  getExistingPreview: (editSession) ->
+    uri = "markdown-preview:#{editSession.getPath()}"
+    for previewPane in rootView.getPanes()
+      previewItem = previewPane.itemForUri(uri)
+      return {previewPane, previewItem} if previewItem?
+    {}